Fix Rect::operator & (intersection)

Completely neglected to normalize the Rect's size. Somehow completely asymptomatic unless looking at the favourites menu if it's too long to fit on the screen, on MacOS. Yeah.

Another in the series of fixes for easily avoidable problems introduced in c2bb777212 by my infinite wisdom >_>. The previous (and thankfully, first) commit in the series is 8cab4ab738.

src/common/Vec2.h

@@ -367,7 +367,8 @@ public:
 	{
 		auto tl  = Vec2<T>(std::max(pos.X         , other.pos.X               ), std::max(pos.Y         , other.pos.Y               ));
 		auto br1 = Vec2<T>(std::min(pos.X + size.X, other.pos.X + other.size.X), std::min(pos.Y + size.Y, other.pos.Y + other.size.Y));
-		return Rect<T>(tl, br1 - tl);
+		auto size = br1 - tl;
+		return Rect<T>(tl, Vec2<T>(std::max(size.X, T(0)), std::max(size.Y, T(0))));
 	}
 
 	inline Rect<T> &operator&=(Rect<T> other)